Well, as i use K9 longer than PM i can't compare, as i never bothered to try the inbuilt one, sorry.Oh, if finally Slaven would announce an upgrade which correctly handled UTF-8 mails, that's the only grugde i have with PM...

I used K9 many years ago, IIRC it was probably better than Pocomail's filters (but it could have been that I have used it before Pocomail had the Bayes filtering...not sure). Since many years ago I use SpamBayes and very happy (same as above, no false positives whatsoever for me with spambayes, that alone is a big timesaver, I don't care about detection rates that much as no false positives are more important given the amount of spam I get). So I think I can recommend both.